1. Field of the Invention
The present invention relates to an image reading apparatus for converting an original image into electrical image data and supplying the image data to a processing apparatus for electrically processing image information, such as a facsimile system, a digital copying machine, or an electronic image file.
2. Description of the Prior Art
An image reading apparatus is known which photoelectrically reads an image on a book or a document or an image recorded on a film or the like, using an image sensor such as a CCD. In such image reading, the density of a read image may vary due to variations in a light source for illuminating a reading object or in other optical members such as a lens and a mirror for guiding light reflected from or transmitted through an object being read to an image sensor, or due to nonuniform sensitivity of a plurality of light-receiving elements of the image sensor.
In view of this, in order to perform image reading with a high reliability, a correction operation called shading correction is performed.
When shading correction is performed, digital image data obtained by A/D conversion of an output analog image signal from a solid-state image sensor is multiplied with or divided by shading correction data.
However, in such a conventional shading correction scheme, the configuration of an operation/processing circuit for performing such multiplication/division becomes complex. In addition, an expensive multiplier or a ROM of a large capacity for storing shading correction data must be used.
In this scheme, a white standard plate of a predetermined density is used. Shading correction data is obtained by reading the white standard plate for all pixels or for every predetermined number of pixels along the main scanning direction by a sensor. Image data of a reading object is read by the same sensor and is corrected by the shading correction data. In this case, if the white standard plate is contaminated, scratched or the like, shading correction data corresponding to such a portion contains an error. Then, the image data which is shading corrected using this shading correction data results in a corresponding spot or stripe in a reproduced image, thus yielding a poor image.